Road surfaces (roads) on which vehicles drive are marked or installed with various road markings such as white lines, yellow lines and cat's eyes constituting traffic lane marking lines. Conventionally the recognition of white lines and other such traffic lane marking lines has, as taught by Patent Reference 1, been done by subjecting an image photographed or taken by image photographing means such as a CCD camera and an image processing ECU to differentiation and binarization processing to detect edges in the image and subjecting point sequences of the detected edge (white line candidate point sequences) to Hough transformation to extract approximated linear components.
Further, when, for example, a white line marked on the road surface is itself physically worn through or partially chipped away, or the white line is indistinct in the photographed image owing to low contrast such as may occur during night driving, the white line cannot be accurately recognized, and, as set out in Patent Reference 2, a technique has been proposed for, in such a case, forming a window in the photographed image, judging the degree of white line wear-off from the density of the white line candidate points in the window, and when wear-off is found, superimposing/compositing a white line candidate point sequences extracted from a photographed image taken a given time earlier on/with the white line candidate point sequences extracted from the current white line image, determining a straight line approximating the composited white line candidate point sequences, and recognizing it as the white line.    Patent Reference 1: Japanese Patent Publication No. Hei 6(1994)-42261    Patent Reference 2: Japanese Laid-Open Patent Application No. 2001-236506
Aside from the above, when compositing photographed images, since the time points at which the images are photographed are different from each other, the brightness of the photographed object (road surface) may differ owing to the effect of shading and the like and if composition should be conducted in a state where the brightness of the road surface portion of one image is brighter than that of the white lines or other such traffic lane marking lines of the other image, there would be a risk of the white lines being buried in the road surface in the composited image that invites the degradation of traffic lane marking line recognition accuracy. However, the prior art set out in the Patent References 1 and 2 has no counter-measures therefore.
Therefore, an object of this invention is to overcome the aforesaid drawbacks and provide a traffic lane marking line recognition system for a vehicle that makes the brightness of the images to be composited equal with each other, thereby enabling accurate recognition of the traffic lane marking line.